1. Field of the Invention
This invention relates to a heater for heating fluids. More particularly, to a heater for water or vaporizing liquefied gases such as liquefied petroleum gas.
2. Description of the Related Art
Fluid heaters for the heating and controlled vaporization of liquefied gases are generally known. One electrically heated liquefied petroleum gas (LPG) vaporizer is disclosed in U.S. Pat. No. 4,255,646. Another liquefied gas vapor unit is disclosed in U.S. Pat. No. 4,645,904. Typically, the vaporizer includes a hollow pressure vessel having a liquefied gas inlet near a lower end and a gas vapor outlet near a closed upper end remote from the liquefied gas inlet. A heating core is typically disposed within the pressure vessel, usually positioned close to the lower end. A plurality of resistive electric heating element may be embedded within the heating core.
Such vaporizers using electric heating elements often require the use of a temperature sensor coupled with a time proportional controller for applying power to the heating elements with a periodic on/off duty cycle determined by the deviation of the core temperature from a predetermined set point. If there is an increase of the core temperature above the set point the control circuit reduces the on time of the duty cycle to reduce the temperature. If the core temperature is below the set point, the control circuit increases the on time of the duty cycle. Complex control circuitry and switches are required.
Such known vaporizer may also have a liquefied gas sensor communicating with the interior of the pressure vessel near its upper end, below the gas vapor outlet. The liquefied gas sensor is typically an overflow sensor or “float switch” for sensing the level of liquefied gas in the pressure vessel and controlling a valve that opens and closes to stop the flow of liquefied gas into the pressure vessel. Accordingly, the valve is controlled to open the pressurized flow of liquefied gas into the pressure vessel and to shut off the flow before the liquefied gas fills the gas vapor head space and liquefied gas floods through the outlet of the vaporizer.
A problem with such known fluid heaters is the need to control the on/off duty cycle of the electric heater elements to prevent overheating. The circuitry required creates safety concerns and, in addition, maintenance and reliability concerns are created. Further, the circuitry increases the cost of manufacturing the fluid heaters.
Another problem common to most known fluid heaters is that, due to the presence of extremely volatile gasses, as well as a combustion source in the form of an electrical circuit and heat source, there is a potential of an explosion. As a result, the fluid heaters must be sufficiently encased in a protective shielding to suppress an internal explosion if they are used in environments that include other combustion or explosion dangers. Such shielding adds expense, weight and size to the fluid heaters, as well as making maintenance issues more complex.